INTRODUCTION The regionalization of the vertebrate hindbrain involves a segmentation process, well conserved through vertebrate evolution (Gilland and Baker, 1993). During early hindbrain development, a series of morphological units, termed rhombomeres

The regionalization of the vertebrate hindbrain involves a segmentation process, well conserved through vertebrate evolution (Gilland and Baker, 1993). During early hindbrain development, a series of morphological units, termed rhombomeres (r), forms along the anteroposterior axis (Hanneman et al., 1988; Lumsden and Keynes, 1989). The rhombomeres serve to organize segmental patterns of neuronal differentiation (Lumsden and Keynes, 1989; Trevarrow et al., 1990; Clarke and Lumsden, 1993) and the ordered migration of cranial neural crest cells into specific branchial arches (Lumsden and Guthrie, 1991; Sechrist et al., 1993; Schilling and Kimmel, 1994). Cell-lineage studies in chick have demonstrated that each of r2-r6 constitutes a lineage-restricted compartment (Fraser et al., 1990; Birgbauer and Fraser, 1994). Members of the vertebrate Hox gene family are expressed in partially overlapping domains in the embryonic hindbrain with anterior expression borders that coincide with distinct segmental boundaries (reviewed by McGinnis and Krumlauf, 1992). The results of targeted gene inactivation of individual Hox genes in mouse revealed that Hox genes are involved in conferring rhombomeric identity (Goddard et al., 1996; Studer et al., 1996), and suggested that at least some Hox genes may play a role in the segmentation process itself (Carpenter et al., 1993; Dollé et al., 1993; Mark et al., 1993; Gavalas et al., 1997). Krox20 (Egr2) (Wilkinson et al., 1989) and kreisler (kr/Mafb) (Cordes and Barsh, 1994) have been shown to be direct transcriptional regulators of distinct Hox genes in the hindbrain. Krox20 is activated before morphologically visible segmentation in the r3 and r5 primordia (Wilkinson et al., 1989; Oxtoby and Jowett, 1993) and directly regulates the expression of Hoxa2 (Nonchev et al., 1996) and Hoxb2 (Sham et al., 1993) in these rhombomeres. Targeted inactivation of Krox20 leads to progressive disappearance of r3 and r5 during development (Schneider-Maunoury, 1993; Swiatek and Gridley, 1993; Schneider-Maunoury et al., 1997). kr is required for specification of r5 and r6 (Frohman et al., 1993; Cordes and Barsh, 1994; McKay et al., 1994) and has been shown to be a direct regulator of Hoxa3 and Hoxb3 in r5 (Manzanares et al., 1997; Manzanares et al., 1999). In contrast to our knowledge of Hox gene regulation, however, relatively little is known about the control of Krox20 and kr expression. Another family of regulatory genes with members expressed in the developing hindbrain is the POU gene family (Ryan and Rosenfeld, 1997). POU genes are expressed in the zebrafish CNS in distinct but overlapping domains during embryonic development (Takeda et al., 1994; Hauptmann and Gerster, 1995; Hauptmann and Gerster, 1996; Spaniol et al., 1996; 1645 Development 129, 1645-1655 (2002) Printed in Great Britain © The Company of Biologists Limited 2002 DEV2750